Ion source for a mass spectrometer



Nov. 27, 1956 L. A. DIETZ 2,772,362

I ON SOURCE FOR A MASS SPECTROMETER Filed April 26, 1955 nited States Patent ION SQURCE FOR A MASS SPECTROMETER Leonard A. Dietz, Pattersonville, N. Y., assignor to General Electric Company, a corporation of New York Application April 26, 1955, Serial No. 504,027 4 Claims. (Cl. 250--41.9)

spatial separation of the ions into separate beams having characteristics mass-to-charge ratios. Ions having the desired mass-to-charge ratio are then selectively discharged at a collector electrode.

The ionization of the gas or gaseous mixture introduced into the mass spectrometer tube is usually achieved by passing an electron beam through the ionization chamber, the resulting ions then being drawn out of the chamber through an exit slit therein. It is extremely important that the cross-sectional width of the electron beam in a plane parallel to that of the slit does not exceed approximately four times the width of the slit in order to minimize the eifects of electron density variation. Ideally, the width of the electron beam should be small relative to that of the Width of the exit slit, however, design considerations and the need for a high differential gas density between the ionization chamber and the rest ofthe spec.- trometer tube make it impractical to reduce the diameter of the electron beam or widen the exit slit, and a four-toone ratio between the two has been found to be a practical one. For further information concerning the. geometry of ion sources for mass spectrometers, reference is made to a copending application of T. E. Usher and the present inventor, Serial No. 413,882, filed on March 3, 1954, under the title of Mass Spectrometer Tube, and assigned to the General Electric Company of Schenectady, New York, the assignee of the present invention.

Although a four-to-one ratio between the diameter of the electron beam and the width of the exit slit is tolerable, some of the ions formed within such an ionization chamher do not pass through the exit slit; instead, they are neutralized within the ionization chamber by striking the surface around the exit slit. This results in a somewhat inaccurate cracking pattern and in a variation in the sensitivity of the mass spectrometer. The present invention is designed to insure that substantially all of the ions formed in back of the exit slit will be pushed through it.

His, therefore, one object of this invention to provide an ionization chamber for a mass spectrometer which will enable the mass spectrometer to have a consistent cracking pattern and sensitivity.

It is another object of this invention to provide an ionization chamber for a mass spectrometer which will insure that substantially all of the ions formed in the ionization chamber will be pushed through an exit slit in the chamber.

Other objects and advantages will appear as the description of the invention proceeds.

Briefly stated, in accordance with the invention, there is disclosed an ionization chamber for a mass spectrometer having a curved ion repeller plate disposed behind a plate with an exit slit opening into the chamber. A potential difference between the ion repeller plate and the exit plate is then created, and the resulting electrostatic lines of force serve to focus the ion beam near the exit slit so as to insure that substantially all of the ions formed pass therethrough. Also, plates may be provided near the edges of the slit to prevent stray ions from collecting near the slit.

The features of this invention which are believed to be novel and patentable are pointed out in the claims which form a part of this specification, For a better understanding of the invention, reference is made in the following description to the accompanying drawing, wherein like parts are indicated by like reference numerals, in which:

Fig. 1 represents a perspective view of mass spectrometer tube embodying the novel ion source of the invention, the tube being broken away in order to show the ion chamber and the chamber itself being shown in cross section; and

Pig. 2 is a cross-sectional view of the ionization chamber of the invention.

Referring now to Fig. 1, there is shown a mass spectrometer tube having an ionization chamber 1 in accordance with the invention, a structure 2 for collecting the ions after they have passed through the body of the tube, and an intermediate analyzer section 3 interconnecting structures 1 and 2. The chamber 1 as shown is contained in an envelope 4, which is broken away to show its details. A pair of pole pieces 5 and 6 are shown disposed on opposite sides of the ionization. chamber 1, and a gas sample introduction port 7 is shown as opening into the ionization chamber.

Disposed around the intermediate analyzer section 3 of the tube is a pair of pole pieces 8 and 9 for providing a magnetic field with which ions from ionization chamber 1 travel and are separated into beams of ions in accordance with their mass-to-charge ratios. The ions are drawn into the intermediate analyzer section by a plurality of slotted plates 1t which are connected to a source of high positive potential (not shown), and the beam of ions having a desired mass-to-charge ratio arrives at a plurality of slotted plates 11 within the collector electrode section 2. The ion beam then falls upon a collector plate 12, which leads to analyzing equipment (not shown) for making a qualitative and quantitative analysis of the ions falling on this plate. For further information concerning mass spectrometers, reference is made to the aforementioned copending application.

Within ionization chamber 1 is shown a portion of a hole 13 through which an electron beam. is sent into the ionization chamber. Also within the ionization chamber is disposed a curved deflector plate 14 in accordance with the invention and a plate 15 having an exit slit 16 therein with a pair of curved plates 17 disposed around this slit, only one plate 17 being shown in this figure.

Reference is now made to Fig. 2 where a cross-sectional view of the ionization chamber of the invention is shown and in which elements 16 and 17 may be more clearly seen. As shown in this figure, the pair of guiding plates 17 are disposed on the sides of the exit slit 16 and are so curved as to be concentric with the curvature of plate 14. As is also shown in this figure, curved repeller plate 14 is connected to the positive terminal of a source of potential 19, the plate 15 with the exit slit 16 therein being connected to the negative terminal of this source of po- Patented Nov. 27, 1956 i 3 tential. It should be understood that the respresentation of the source of potential 19 is only symbolic since, in practice, a suitable potential would be provided from a voltage divider within a power supply which would also provide suitable potentials for all of the plates in the mass spectrometer.

It is apparent from Fig. 2 that the curved repeller plate 14 is positive relative to the exit slit plate 15, and this difference in electrical potential causes lines of equal potential 18 to be produced between exit slit 16 and repeller plate 14 with the configuration shown. It is well known in the art that the lines of electrostatic force existing between two charged bodies must always be at right angles to the lines of equal potential, and these lines of force have been so depicted by lines 20.

By properly designing the curvatures of repeller plate 14- .and guiding plates 17, it is possible to insure that the lines of force will be focused at a point 21 in the region of the exit slit 16 and that the two outermost lines of force will be at the boundaries of the electron beam 13, all other lines of force failing to have any effect upon the ions since they are effectively blocked by guiding plates 17.

In the operation of the novel ionization chamber of the invention, a gas sample is caused to enter the ionization chamber through the duct 7 and a stream of electrons is also caused to traverse the ionization chamber through the hole 13. The ions occurring due to the electron tream are then focused by the electrostatic lines of force 20 so that they are forced through the slit 16 in plate 15 and into the body of the mass spectrometer.

The pair of curved guide plates 17 on either side of the slit 16 serve to prevent any stray ions from collecting around the slit 16 and also serve to improve the focusing action of the electrostatic lines of force. Once the ions have been caused to pass through the slit 16, they are accelerated and separated in the body of the mass spectrometer tube in accordance with the usual procedures prevailing in the art.

In building an ion source for a mass spectrometer tube in accordance with the embodiment shown in Fig. 2, the inner radius of the repeller plate 14 was chosen to be 0.140" as measured from the focal point 21 of the electrostatic lines of force, the distance between the center of the electron beam hole 13 and the top of slit 16 being 0.070", and the inner radius of guide plates 17 was chosen as 0.040" and the outer radius as 0.060. Voltage source 19 was chosen so as to provide a difference in potential between repeller plate 14 and plate 15 that is of the order of 8 volts. These are only approximate values and dimensions, and the most desirable values can readily be determined by experiment with individual mass spectrometers.

An ion source constructed in accordance with the invention and used in a mass spectrometer tube results in greater stability of the sensitivity and the cracking patterns since a higher proportion of the ions pass through the exit slit and fewer ions collect around the edges of the exit slit. Also, there is an increase in sensitivity in the mas spectrometer which is as much as twice that presently known in the art. Further, this increase in sensitivity and stability is achieved without any extensive change in the design of the instrument, since no new electrical leads are required in incorporating the novel repeller plate and exit slit guidance plates into the ionization source of a mass spectrometer.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention; and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An ion source for a mass spectrometer tube comprising, an ionization chamber having a slit at one end through which ions may pass, a curved ion repeller plate disposed within said ionization chamber opposite the slit in the one end of said chamber and having a positive potential relative to said one end of said chamber for electrostatically focusing any ions formed within said chamber at a point in the region of said slit and a pair of guiding plates disposed on the sides of said slit for guiding ions through said slit.

2. In combination with a mass spectrometer tube, an ion source comprising, an ionization chamber having a slit at one end through which ions may pass, a curved ion repeller plate disposed within said ionization chamber opposite the slit in the one end of said chamber and having a positive potential relative to said one end of said chamber for electrostatically focusing any ions formed within said chamber at a point in the region of said slit, and a pair of guiding plates disposed on the sides of said slit for guiding ions through said slit in order that they may enter into the body of said mass spectrometer tube.

3. An ion source for a mass spectrometer tube comprising, an ionization chamber receptive of gas samples and having a slit at one end through which ions may pass, an ion repeller plate disposed within said ionization chamber opposite the slit in the one end of said chamber, said chamber having a hole therein through which an ionizing beam of electrons may pass between said ion repeller plate and said slit, said ion repeller plate having a positive potential relative to said one end of said chamber and being so curved that lines of force emanating therefrom focus any ions formed within said chamber at a point in the region of said slit, and a pair of guiding plates disposed on the sides of said slit for blocking any lines of force between said repeller plate and said slit except those passing through said beam of electrons.

4. In combination with a mass spectrometer tube, an ion source comprising, an ionization chamber receptive of gas samples and having a slit at one end through which ions may pass, an ion repeller plate disposed within said ionization chamber opposite the slit in the one end of said chamber, said chamber having a hole therein through which an ionizing beam of electrons may pass between said ion repeller plate and said slit, said ion repeller plate having a positive potential relative to said one end of said chamber and being so curved that lines of force emanating therefrom focus any ions formed within said chamber at a point in the region of said slit in order that they may enter the body of said mass spectrometer tube, and a pair of gliding plates disposed on the sides of said slit for blocking any lines of force between said repeller plate and said slit except those passing through said beam of electrons.

References Cited in the file of this patent UNITED STATES PATENTS 

